Flat-panel projection display

ABSTRACT

A flat-panel projection display comprises a slab waveguide having a preferably embossed diffraction grating on one face, a lens for directing light into an edge of the waveguide, and in the focal plane of the lens a liquid-crystal modulator for modulating the intensity of the light as a function of lateral position and elevational direction of travel. The light is ejected from the slab waveguide by the grating at angles corresponding to the input angles, giving a virtual display. The light from the modulator can be expanded in one dimension by passing through a magnifying waveguide, followed by scattering in the plane by a screen and projection by a lens at the other end of the waveguide. Head-up and 3-D displays can be constructed using this principle.

FIELD OF THE INVENTION

[0001] This invention relates to 3D displays, head-mounted displays andother compact projection displays.

BACKGROUND OF THE INVENTION

[0002] Projection displays conventionally comprise a two-dimensionalarray of light emitters and a projection lens. The lens forms an imageof the array at some plane in space, and if this imaging plane is farfrom the projection lens, so that the light rays are more or lessparallel, then the effect of the projection lens is essentially tocollimate light from any pixel on the two-dimensional array.

[0003] Projection displays are most commonly configured so that theimage of the array falls on a large translucent screen, and a viewerlooking at the screen will see a greatly magnified image of the pictureon the two-dimensional array. However, it is becoming increasinglycommon for small projection displays to be mounted on the head of aviewer so that the projection display is directed towards the viewer'seye, and light collimated by the projection lens from a single pixel onthe two-dimensional array of light emitters is subsequently focused bythe viewer's cornea onto the retina so that the viewer sees anapparently distant image often known as a virtual image.

[0004] It is also possible to place a large-diameter projection displaycomprising a two-dimensional array of directional light emitters behinda liquid-crystal display or some other spatial light modulator in orderto synthesize a three-dimensional image. See, for example, Travis, A. R.L., “Autostereoscopic 3-D display”, Applied Optics, Vol. 29, no. 29, pp.4341-3. One pixel at a time of the two-dimensional array of lightemitters is illuminated, and an appropriate view of a three-dimensionalobject is simultaneously displayed on the liquid-crystal display in sucha way that the view of the three-dimensional object is only visible ifobserved from the direction in which the rays of light collimated by theprojection lens from the pixel are traveling. A sequence of views isrepeated at a rate faster than that at which the eye can detect flicker,thereby time-multiplexing a three-dimensional image.

[0005] This display is three-dimensional but not holographic. It ispossible in principle to create a holographic three-dimensional image byplacing a two-dimensional array of point-source light emitters in thefocal plane of the projection lens, illuminating each point source inturn, and displaying appropriate holograms on a liquid-crystal displayplaced on top of the projection lens so that each hologram is madevisible to a different point of view in turn.

[0006] Head-mounted displays are bulky and users would prefer that theywere flat. A head-mounted display can be made flatter using a slabwaveguide incorporating a weak hologram, as shown by Amitai, Reinhornand Friesem, “Visor-display design based on planar holographic optics,”Applied Optics, Vol. 34, No. 8, pp. 1352 to 1356, 10 Mar. 1995. Lightfrom a cathode-ray tube and hologram is coupled into the waveguide, andthis light will be diffracted out of the waveguide (i.e. normal to theslab) by the hologram in directions which are determined by the pixelwithin the cathode-ray tube from which the light was emitted.

[0007] Three-dimensional images synthesized by time-multiplexing theillumination of a liquid-crystal display require the liquid-crystaldisplay to have a fast-switching array of thin-film transistors andthese are expensive. Trayner and Orr in U.S. Pat. No. 5,600,454 describea device which avoids this by placing a hologram behind a conventionalliquid-crystal display that directs the illumination of alternate rowsto a left-eye or right-eye view. But both this and theswitched-illumination concept are bulky, and do not exhibit the flatnessneeded for head-mounted displays.

[0008] Instead, a flat-panel three-dimensional display can be made bycombining a projection display with a screen from which light shoneparallel to the surface of the screen is ejected at one of a set ofselectable lines along the screen, as described in the inventor'searlier application PCT/GB 97/02710 (WO 98/15128). One line at a time onthe screen is selected, and simultaneously the projection displayprojects a line of pixels parallel to the screen so that they areejected at the selected line. The same line of pixels on the projectiondisplay is altered repeatedly as each of the series of lines on thescreen is selected in turn in such a way as to time-multiplex a completeimage on the screen. Only one line of the projection display is used, sothe array of light emitters need be only one line high, and if theemitted light is collimated in the plane of the screen then theprojection lens need be only one or two millimeters high so that thecombined projector and screen are flat.

[0009] If it is light from a three-dimensional display, albeit one whosearray of light emitters is only one pixel high, that is directedparallel to the surface of the screen of selectable lines, then theimage formed on the screen is three-dimensional. The three-dimensionaldisplay might comprise an array of light emitters behind a projectionlens with a liquid-crystal display in front of the projection lens, asdescribed above, but in order to put up several views within one lineperiod of the display the switching rate of the liquid crystal wouldneed to equal the number of views times the line rate of the display,and few liquid-crystal mixtures switch this fast.

[0010] Many other kinds of autostereoscopic and holographicthree-dimensional display concepts exist and any could be used in aflat-panel system. Particularly interesting is an old concept comprisinga group of small video projectors in the focal plane of a field lens.Each projector is positioned to form a view in the plane of the fieldlens just as if the lens were a translucent screen, but unlike atranslucent screen the field lens collimates the light so that thepicture is visible from only a single direction. The other projectorsform views which are made visible by the field lens to other directionsso that the viewer sees an autostereoscopic three-dimensional image.However, viewers prefer three-dimensional images to be autostereoscopicboth in azimuth and in elevation, and little consideration has beengiven with this concept to making views vary with elevation.

SUMMARY OF THE INVENTION

[0011] According to the present invention there is provided a flat-panelprojection-display comprising a slab waveguide having a preferablyembossed diffraction grating on one face, a lens for directing lightinto an edge of the waveguide, and in the focal plane of the lens meansfor modulating the intensity of the light as a function of lateralposition and elevational direction of travel.

[0012] This arrangement converts pixels from the modulating means intoplane waves impinging at different angles on the waveguide, which arethen diffracted at corresponding angles out of the face of thewaveguide. A virtual image is thus formed on the waveguide, which can beused, for instance, for head-up displays or 3-D displays.

[0013] To inject more light into the waveguide the display preferablyincludes a one-dimensional screen for spreading the light over the widthof the edge of the slab. This screen can itself be embodied in awaveguide, and mirrors can be associated with the end or ends of thewaveguides to retain the light. The input waveguide can also magnify theinput image. The display can be folded by the use of prismatic waveguideends. Waveguide lenses are also envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For a better understanding of the invention, specific embodimentswill now be described by way of example with reference to theaccompanying drawings, in which:

[0015]FIG. 1 illustrates a mirror angled to reflect vertically incidentlight through 90°, representing background to the invention;

[0016]FIG. 2 illustrates a series of mirrors similar to the singlemirror of FIG. 1;

[0017]FIG. 3 illustrates how the direction of light coupled out of aslab waveguide by a grating embossed on one face of the slab waveguideis determined by the direction of light injected into the waveguide andthe periodicity of the grating;

[0018]FIG. 4 illustrates a flat-panel projection display embodying theinvention;

[0019]FIG. 5 illustrates a flat-panel projection display with a largescreen, the image being magnified from a microprojector;

[0020]FIG. 6 illustrates a version of the display shown in FIG. 5 whichuses a one-dimensionally translucent screen that is reflective insteadof transmissive;

[0021]FIG. 7 illustrates a folded version of the display shown in FIG. 6with the screen curved round the viewer so as to address the viewer'speripheral vision;

[0022]FIG. 8 illustrates a row-and-column-multiplexed flat-panelprojection display;

[0023]FIG. 9 shows how a prism can convert in-plane variations in theray direction (k_(parallel)) into out-of-plane variations in raydirection (k_(transverse));

[0024]FIG. 10 shows how prisms can be used to fold a flat-panelprojection display;

[0025]FIG. 11 is a blown-up view of a folded row-and-column-multiplexedflat-panel projection display;

[0026]FIG. 12 is a compact view of a folded row-and-column-multiplexedflat-panel projection display and shows how the liquid-crystal displayis at 45° to the plane of the flat panel;

[0027]FIG. 13 illustrates a flat-panel three-dimensional display;

[0028]FIG. 14 illustrates flat-panel illumination of a three-dimensionaldisplay using a reflective liquid-crystal display such as an opticallyaddressed spatial light modulator;

[0029]FIG. 15 illustrates a flat-panel scanning-line three-dimensionaldisplay; and

[0030]FIG. 16 illustrates how a variable-thickness waveguide can be usedto act as a lens.

DETAILED DESCRIPTION

[0031] It is a simple experiment to shine light vertically up at amirror pivoted at 45° to the horizontal and FIG. 1 shows how the mirrorreflects the light into the horizontal plane (dotted line). If thedirection of the incident light is rotated in the vertical plane awayfrom the initial direction of the reflected light, then the direction ofthe reflected light is rotated in the horizontal plane. Similarly if thedirection of the incident light is rotated in the plane shared by theincident and reflected light, then the direction of the reflected lightis rotated in the same vertical plane through an equal angle.

[0032] Mirrors can be made partially reflective, and a series of mirrorspivoted at 45° can be stacked one on top of another as shown in FIG. 2so that light shone vertically up at the bottom mirror and not reflectedby that mirror will pass through successive mirrors of the stack untilthe light is all reflected. The series of mirrors behaves in the sameway as a single mirror in that if the direction of the incoming light isrotated in the appropriate vertical plane, then the direction of thereflected light is rotated in the horizontal plane. If these mirrors aresufficiently wide and thin and if there are enough of them then theresult is a flat panel device from all parts of whose surface light canbe made to travel in any single, selectable horizontal direction. Onecannot rotate the direction of the incident light in the plane shared byincident and reflected light without causing the light to stray from theplane of the stack of mirrors, but if the stack is encapsulated in aslab waveguide then the waveguide will keep the light confined to thestack and it will become possible to rotate the direction of thereflected light in the vertical plane. However, a ray bouncing betweenthe two walls of a slab waveguide will travel in either of twodirections alternately, so the stack of mirrors will eject raystraveling in two different directions.

[0033] Another way of ejecting a ray of light propagating in a slabwaveguide normal to the surface of the waveguide is to have a grating ofappropriate spatial frequency embossed on one surface of the waveguide,as shown in FIG. 3. Just as with the mirrors of FIG. 1 and FIG. 2,rotating the direction of the guided ray by an angle θ about an axisnormal to the surface of the waveguide will cause the direction of theejected ray to rotate through the same angle in the plane shared by thenormal and any line of the grating. As FIG. 3 shows, it is also possibleto make the direction of the ejected ray alter in the orthogonaldirection by altering the angle of the propagating ray about an axisparallel to any line of the grating. The ray only interacts with thegrating during reflection, so rays will be ejected traveling in only onedirection (provided that the grating is blazed, or the direction of theother first diffracted order is within the critical angle).

[0034] Suppose that the wavelength of the ray is λ, the grating spacingis d, the direction of the grating periodicity is j, the normal to theplane of the grating is I and the third direction is k. If the ray oflight is incident on the grating at an arbitrary angle, and angles φ, ψand θ are as depicted in FIG. 3, then the wave-vector of the incidentlight, β_(in), can be expressed as:$\beta_{i\quad n} = {{\frac{2\quad \pi}{\lambda}\cos \quad {\varphi ( {{\cos \quad \psi \quad i} + {\sin \quad \psi \quad j}} )}} + {\frac{2\quad \pi}{\lambda}\sin \quad \varphi \quad k}}$

[0035] The wave vector of the first-order diffracted ray, β_(out), willbe:$\beta_{out} = {{\frac{2\quad \pi}{\lambda}\cos \quad {\varphi ( {{\cos \quad \theta \quad i} + {\sin \quad \theta \quad j}} )}} + {\frac{2\quad \pi}{\lambda}\sin \quad \varphi \quad k}}$

[0036] φ is the same for both input and output, so the angle of azimuthat which the ray leaves the grating is uninfluenced by the ray's angleof elevation. But θ is different from ψ and this leads to distortion inthe other axis.

[0037] The flat-panel projection display illustrated in FIG. 4represents an embodiment of the invention using the above principle. Itcomprises a slab waveguide 1, a weak diffraction grating 2 embossed onthe slab waveguide, a lens 3, a liquid-crystal display 4 illuminatedpreferably with collimated light, a one-dimensionally translucent screen5 and a front-silvered mirror 6. One end of the slab waveguide 1 isplaced in one focal plane of the lens 3, and the liquid-crystal display4 is placed in the other focal plane of the lens 3, so that light fromany pixel on the liquid-crystal display 4 will be collimated into aplane wave, part of which will enter the end of the slab waveguide 1.One face of the slab waveguide 1 is embossed with a weak diffractiongrating 2 such that as the wave propagates down the waveguide 1 part ofit is continually diffracted out of the waveguide: Diffracted componentsof the wave emerge from all parts of the diffraction grating 2 andcombine into a single wavefront whose direction is determined by thepixel on the liquid-crystal display 4 through which the light passed.Waves traveling in other directions are modulated by other pixels on theliquid-crystal display 4, with the result that a complete(two-dimensional) virtual image is projected from a slim, flat waveguide1.

[0038] For greater efficiency it is preferable that all the light fromthe liquid-crystal display 4 is injected into the end of the slabwaveguide 1. To this end the illumination of the waveguide 1 comprisescollimated rays which are passed through the one-dimensionallytranslucent screen 5 adjacent to the liquid-crystal display 4. Thescreen 5 might comprise for example an array of small cylindricallenslets which diffuse the rays over a range of angles in one (thevertical) dimension but leave them collimated in the other dimension sothat in the other focal plane of the lens 3 the whole of the end of theslab waveguide 1 is illuminated.

[0039] It is also preferable that light coming from one pixel of theliquid-crystal display 4 is injected into only one mode of the slabwaveguide 1. This requires that as the plane wave is injected into theend of the slab waveguide 1 there is also injected a plane wave of equalintensity having the same component of direction resolved in the planeof the waveguide 1, but the opposite component of direction resolvedperpendicular to the waveguide 1. In terms of a ray description thepurpose of this second wave is to fill in the gaps on the front of theslab waveguide 1 which would otherwise be left unilluminated by theoriginal wave. The second wave can be provided by placing the front of afront-silvered mirror 6 against the front of the slab waveguide 1 sothat the mirror 6 protrudes beyond the end of the waveguide 1. Lightfrom the liquid-crystal display 4 must be sufficiently diffuse in bothdimensions to illuminate both the end of the waveguide 1 and its imagein the mirror 6; this can be done either by adding a second weakone-dimensionally translucent screen orthogonal to the first, or bymaking the pixels of the liquid-crystal display 4 small enough to causethe diffusion by diffraction.

[0040] One class of projection display, the head-up display, is commonlyfound in aircraft and comprises a large (several inches in diagonal)screen from all parts of which is projected a (virtual) image whichcomes into focus in the far field. The flat-panel projection displaydescribed above could be configured to make such a head-up display, butthe lens 3 and liquid-crystal display 4 would be inconveniently large.FIG. 5 shows how the image from a small liquid-crystal display 4 can bemagnified by projection within a second slab waveguide 7 of similardimensions to the display waveguide 1. The liquid-crystal display 4 isplaced in one plane of a projection lens 3 and the end of this slabwaveguide 7 in the other, and the liquid-crystal display 4 isilluminated by collimated light. Rays from a single row of theliquid-crystal display 4 have a direction within the slab waveguide 7which, resolved in a plane normal to the liquid-crystal rows (left toright in the diagram), have a single angle (sometimes called theout-of-plane angle). However, rays from a single column of theliquid-crystal display 4 are projected to a single zone of the end ofthe magnifying waveguide 7, and a one-dimensionally translucent screen 8is placed at the end of the slab waveguide 7 so as to preserve theout-of-plane angle of the rays, but to scatter their angle in the planeof the slab waveguide 7 (sometimes called the in-plane angle). This ineffect produces a one-dimensionally magnified real image at the screen8.

[0041] The rays are then coupled into a third length of slab waveguide 9at the end of which is a cylindrical lens 10, preferably integral withthe waveguide itself, whose axis is orthogonal to the plane of the slabwaveguides. The one-dimensionally translucent screen 8 is to be in thefocal plane of the cylindrical lens 10 so that rays from any point onthe screen 8 are collimated as they leave the lens 10. Instead of a lensa mirror could be used, with appropriate re-configuration of thewaveguides; a mirror in fact gives rise to less distortion.

[0042] The light is then passed into the slab waveguide 1 embossed witha weak diffraction grating 2, and diffracted out, as above, to give afar-field-projected image. In order to confine rays to the sameout-of-plane angle throughout, the one-dimensionally translucent screen8 and cylindrical lens 10 are made with the same thickness as the slabwaveguides, and the fronts of a pair of front-silvered mirrors 11, 12are placed above and below each of the interface elements 8, 10 so as toconfine rays to the same out-of-plane angle. The one-dimensionallytranslucent screen 8 can be formed for example of an array ofcylindrical lenslets.

[0043] The large flat-panel projection display of FIG. 5 is long, and itis difficult to cut and polish the array of cylindrical lenslets used toeffect the one-dimensionally translucent screen 8 to the same thicknessas the adjacent slab waveguides 7, 9 within optical tolerances. FIG. 6shows how matters can be improved by using a translucent orone-dimensionally specular mirror 13 instead of a translucent screen 8.The one-dimensionally translucent mirror is simply an array ofcylindrical lenslets coated with aluminium, and this can be placed closeenough to the end of the magnifying slab waveguide 7 that despite theabsence of front-silvered mirrors 11, 12 there is minimal loss of rayconfinement during reflection off the translucent mirror 13. Here theone waveguide 7 serves both for magnification and for collimation byvirtue of the double pass of the light through it.

[0044] The flat-panel projection display of FIG. 6 is still rather long.It is well known that a bend with a small radius of curvature in awaveguide will alter the out-of-plane angle of a ray, but a bend with asufficiently large radius of curvature will not disrupt rays and it hasbeen found by experiment that a radius of curvature of 5 cm is notdisruptive. FIG. 7 shows how a bend 7 a can be introduced to fold thesystem of FIG. 6 in effect folding the waveguide 7 back behind thescreen 1. In FIG. 7 the screen 1 is also curved about a vertical axis sothat a viewer sitting near the center of the curve will see pictureswith his or her peripheral vision. Curving the screen 1 without furthermodification will cause rays from a single pixel to converge instead ofbeing collimated as required. The solution is to move theone-dimensionally translucent mirror 13 closer to the cylindrical lens10; there will be a distance where the convergence lost by doing thiswill cancel out the extra convergence caused by curving the screen.Although head-up displays are commonly used in aircraft, it is thoughtthat this design of display will be sufficiently cheap for very large(perhaps a couple of meters diagonal) displays to be built, and that thedisplays might be used in offices either to display virtual-realityimages, or as a screen more comfortable for the long-sighted viewer.

[0045] A second class of projection display, the head-mounted display,is commonly used to display virtual-reality images, but existingdisplays are bulky and grotesque. Users would prefer a display to beflat and slim like a pair of sunglasses, but while all the displays sofar described have flat slab waveguides, the projector is relativelybulky. Liquid-crystal displays can be miniaturized, but it is difficultto make liquid-crystal pixels smaller than two or three microns, and theresulting display is still too big.

[0046]FIG. 8 shows how two one-dimensional liquid-crystal displays ordevices can be used to synthesize a projected image in a flat device.The first liquid-crystal display 14 is configured as a grating and whenilluminated with collimated light will diffract the light in twodirections at equal but opposite in-plane angles to the central axis.The light then passes by way of a pair of lenses 17, 18 (whose functionis described below) through a micro-prism 15, shown in detail in FIG. 9,as having three cubes with successive 45° mirrors, which rotates theincoming light by 90° so as to convert in-plane changes of ray directionto out-of-plane changes of ray direction. The light is also reversed indirection but for convenience whits is not shown in FIG. 8.

[0047] The rays are then expanded by a cylindrical lens or mirror 16 ato illuminate the whole of a one-dimensionally translucent screen 8adjacent to the second one-dimensional liquid-crystal display 16. Thesecond liquid-crystal display 16 is in the focal plane of a finalcylindrical lens 10, and modulates the in-plane angles of light enteringthe final slab waveguide 1. Rays at each angle are converted by the weakdiffraction grating embossed on the slab waveguide 1 into columns in thefar-field-projected image. The first liquid-crystal display 14 modulatesthe out-of-plane angle of all rays entering the final slab waveguide 1,which is converted by the weak diffraction grating 2 into a row in thefar-field image. For each out-of-plane angle selected by the firstliquid-crystal display 14, the second liquid-crystal display 16modulates all in-plane angles, and a far-field-projected picture iswritten line by line in much the same way as in a cathode ray tube.

[0048] Although liquid-crystal pixels can be made with dimensions of 2or 3 microns, it is easier to make pixels with dimensions of 20 or 30microns, but the maximum angle of diffraction achievable with suchpixels is approximately 1°. The two lenses 17, 18 between the firstliquid-crystal display 14 and the micro-prism 15 magnify this maximumangle of diffraction to 10° or more. The first of the two lenses 17 hasa focal length at least ten times greater than the second 18, and theyshare a focal plane so that at the micro-prism 15, which is in theunshared focal plane of the second lens 18, rays enter the prism 15 at aconstant point but from a variety of angles.

[0049] If the first liquid-crystal display 14 modulates amplitude in theconventional manner, then light will be transmitted in the zero(undiffracted) order and in the second and higher diffracted orders aswell as in the first order. The second and higher diffracted orders canbe minimized by choosing a suitable grating pattern, while the zeroorder can be eliminated by placing opaque material at the center of thefocal plane shared by both lenses 17, 18. The zero order will not ariseif the liquid-crystal display 14 comprises ferroelectric liquid crystalconfigured to modulate the phase of light by 0° and 180°, and since bothliquid-crystal displays 14, 16 may need to be ferroelectric in order toswitch sufficiently quickly this alternative may be preferable.

[0050] It remains to fold the system of FIG. 8 into an area the samesize as a spectacle lens, but bending the waveguide through a radius ofcurvature of 5 cm would make the result much too bulky. Instead FIG. 10shows how a pair of right-angled prisms 19 can be used to fold thewaveguide; such an arrangement can also be used for the previousembodiments. A low-refractive-index material must be placed on bothtransmissive surfaces of each right-angled prism 19 in order to keeprays confined to the correct in-plane angle, and the 45° surface of eachright angled prism 19 must be silvered in order that light is alsoreflected at acute angles.

[0051]FIG. 11 shows an expanded view of each layer of the folded systemof FIG. 8, and FIG. 12 shows the compressed system. The twoliquid-crystal displays 14, 16 are parts of a single liquid-crystaldevice shown in FIG. 12 whose liquid-crystal layer is sandwiched betweena single glass or silicon substrate and a transparent top layer whichmight be made of Mylar and is kept as thin as possible in order to avoidthe in-plane angle of rays being corrupted, or the rays being allowed toescape.

[0052] In this device light enters from the side of the first slab, andis scattered or expanded by a front-silvered cylindrical mirror 23 toform a plane wave which is then split by the grating LCD 14 and sent upto the next slab by a first prism pair 19 a. Here the light is convergedto a corner equivalent to the micro-prism 15 via a lens 18 analogous tothe lens 3 in FIG. 5, and is sent up to the third slab, the “magnifying”slab 7, with the one-dimensionally translucent lens 8 and the second LCD16. The out-of-plane reflections start in the third slab, as shown. Thefourth slab 9 is the cylindrical collimating lens 10 and the final slabis the display or output slab 1.

[0053] A third class of projection display, the three-dimensionaldisplay, is finding uses in amusement arcades and operating theaters,but existing displays are too bulky. Just as a three-dimensional displaycan be made by placing a large projection display behind ahigh-frame-rate liquid-crystal display, FIG. 13 shows how a flatthree-dimensional display can be made by placing behind a liquid-crystaldisplay 20 a flat-panel projection display such as that shown in FIG. 5.Here the slab functions simply as a source of collimated light scannedthrough the range of output angles by transverse movement or scanning ofthe light source transversely, in synchrony with the modulator 20, asshown by the arrows. For a 3-D display no modulation in elevation isneeded and the grating need not be a regular diffraction granting butcan simply be a set of parallel scattering lines. The frame rate ofliquid-crystal displays is limited by the conductivity of the patternedlayer by which a voltage is placed across each pixel, and theconductivity of the layer can be increased if it is made so thick thatit is opaque. For example, a microdisplay can be made with a frame rateof 20 kHz by placing a layer of ferroelectric liquid-crystal over acrystalline silicon integrated circuit, but the liquid-crystal displaywill then only work in reflection.

[0054] Another way of making a liquid-crystal display with a high framerate is to project an image onto the photosensitive side of an opticallyaddressed spatial light modulator, but this device also only works inreflection. FIG. 14 shows how a flat-panel projection display can beused to provide front illumination by the use of a slab waveguide 1 witha volume hologram or a stack of partial mirrors at 45° to the plane andaxially parallel to the entry edge of the slab 1. Mirrors—e.g. a stackof glass slides—are preferable for HUD applications because they do notcause spurious diffraction of sunlight into the user's eyes, as ahologram tends to do. The transversely modulated light from the source 4is directed into the slab 1 by the lens 3. It is reflected backwards, bythe partial mirrors or the hologram, towards the LCD 20 which is ineffect an array of mirror pixels. These mirror pixels are modulated by aprojector 30 to be on or off, depending on the image at the particularangle of view that is currently being produced. The image then passesback though the slab 1, where nearly all of it is transmitted, to theviewer.

[0055] A flat-panel projection display can also be used tofront-illuminate a still hologram by configuring the display toilluminate the hologram with waves collimated in a single direction. Thetechnique can also be extended to provide flat-panel illumination of awide-field-of-view holographic video display by using the flat-panelprojection display to illuminate a liquid-crystal display 20 with wavescollimated in one at a time of several discrete directions,simultaneously writing appropriate holograms on the liquid-crystaldisplay, and repeating the sequence within the flicker response time ofthe eye.

[0056] It is difficult to deposit on large screens the thin-filmtransistors needed for high-frame-rate liquid-crystal displays, and WO98/15128 describes how a three-dimensional display can be made insteadby shining light parallel to the surface of a screen which incorporatesa set of lines any one of which will eject the light from the screenwhen it is selected. But that document explains only how such a devicecan produce images which are three-dimensional in azimuth. For truethree-dimensional or virtual-reality images it is necessary to controlthe intensity of a ray leaving any point of the screen as a function ofits direction both in azimuth and in elevation.

[0057]FIG. 15 shows how a three-dimensional display with control ofelevation is made by combining a projection display with a screen 21 onwhich light shone approximately parallel to the surface of the screen 21is ejected at one of a set of selectable lines along the screen 21. Thisgenerally follows the scheme set out in the inventor's earlier WO98/15128. But now the screen 21 is configured as a slab waveguide, andall lines of the projection display are modulated, so that there iscontrol of both the in-plane angle and the out-of-plane angle of rayswithin the slab waveguide. The cladding of the slab waveguide herecomprises nano-droplet polymer-dispersed liquid-crystal which is amaterial whose bulk refractive index can be controlled with a voltage.It is pixellated into a series of lines so that light will be ejectedfrom the slab waveguide at the line where the refractive index of thecladding has been reduced. The ray direction in azimuth will becontrolled by the in-plane direction of the rays within the slabwaveguide, but the ray direction in elevation will be determined by theout-of-plane direction of the rays. If the projection display 22constitutes a video hologram, then the pattern of light ejected at theselected line will also constitute a hologram, and this gives a way ofscreening a video hologram which is three-dimensional both in azimuthand in elevation without the need for thin-film transistors. However,the field of view of the hologram in elevation will be narrow. Widerfields of view in elevation can be created by using a switchableliquid-crystal grating to eject the light, varying the spatial frequencyof the liquid-crystal grating so as to project one-by-one severalholograms to different angles in elevation from each line.

[0058] Several of the embodiments described so far have required the useof a cylindrical lens between adjacent waveguides, but including thelens as a separate element requires that the surfaces between waveguideand lens have to be polished, and this is costly. FIG. 16 shows how theout-of-plane angle of a ray in a waveguide will become greater if thewaveguide becomes gradually thinner. If the out-of-plane angle of theray is large, then the component of ray velocity in the plane of thewaveguide decreases. Just as lenses focus collimated light by havingthick centers so as to slow the central part of the wave with respect tothe periphery, a waveguide can be made to focus a collimated wave bymaking the central part of the waveguide thinner than the edges. Such“lenses” can be used in all the described embodiments, and indeed in anyflat optical system requiring a lens.

[0059] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures as is permitted under the law.

1. A flat-panel projection display comprising a waveguide systemincluding a display slab waveguide having a grating acting to causelight propagating in the waveguide to emerge from one face of thewaveguide, a lens element for directing light into an edge of thewaveguide system, and in the focal plane of the lens element means formodulating the intensity of the light as a function of lateral positionand of elevational direction of travel, relative to the waveguide, thegrating being such as to eject light directed into the waveguide systemfrom the slab towards a viewer at an angle depending on the said lateralposition and elevational direction.
 2. A display according to claim 1,in which the modulating means includes a liquid-crystal modulator.
 3. Adisplay according to claim 1, further including a screen for spreadingthe light in the waveguide system one-dimensionally over the width ofthe edge of the slab.
 4. A flat-panel projection display according toclaim 3, in which the modulator and lens element are smaller in thelateral direction than the slab is, and the waveguide system includes amagnifying slab waveguide of matching width placed between the lenselement and the display slab waveguide so as to expand the light fromthe lens to the full width of the display waveguide.
 5. A flat-panelprojection display according to claim 4, in which the screen is aone-dimensionally translucent strip at the end of the magnifyingwaveguide, conserving out-of plane angle but spreading light in theplane.
 6. A flat-panel projection display according to claim 4, in whichthe waveguide system includes an intermediate slab waveguide couplingthe light from the magnifying waveguide into the display waveguide.
 7. Aflat-panel projection display according to claim 6, in which theintermediate waveguide is the same as the magnifying waveguide, thelight passing though the waveguide in one direction for magnifying, andpassing back in the reverse direction for coupling into the displaywaveguide.
 8. A flat-panel projection display according to claim 7, inwhich the screen is a one-dimensional cylindrical mirror strip at theend of the magnifying waveguide, conserving out-of plane angle butspreading light in the plane.
 9. A flat-panel projection displayaccording to claim 6, in which the waveguide system includes a lens ormirror associated with the intermediate waveguide for creating afar-field (parallel-ray) image entering into the display waveguide. 10.A flat-panel projection display according to claim 4, in which any orall of the slabs are coupled in the out-of-plane direction by mirrorscovering the line between adjacent slabs.
 11. A flat-panel projectiondisplay according to claim 9, in which the magnifying waveguide; and theintermediate waveguide are folded over to lie behind the displaywaveguide.
 12. A flat-panel projection display according to claim 1, inwhich the modulating means includes two one-dimensional modulators, thefirst modulating input light at desired angles corresponding toout-of-plane angle in the final image, and the second modulating thein-plane angles for each out-of-plane angle of the final image.
 13. Aflat-panel projection display according to claim 12, in which the planeof light emerging from the first modulator is tumed by a microprism sothat the entire apparatus is essentially flat.
 14. A flat-panelprojection display according to claim 12, in which the output of thefirst modulator is expanded by a cylindrical optical device so that itilluminates the whole of a one-dimensionally translucent screen adjacentto the second modulator.
 15. A flat-panel projection display accordingto claim 13, in which the waveguide system comprises slab waveguidesincorporating the optical elements.
 16. A flat-panel projection displayaccording to claim 15, in which the waveguides are stacked, the lightfrom one to the next being coupled at adjacent ends by prisms, and thetwo modulators are provided on a single substrate.
 17. A flat-panelprojection display according to claim 12 and constituting a head-updisplay.
 18. A flat-panel projection display according to claim 1, inwhich the slab is itself modulatable, so that at any one time light fromonly one row is ejected, a set of one-dimensional modulators providingthe input light at the various in-plane angles.
 19. A flat-panelprojection display comprising a waveguide system including a displayslab waveguide having a grating acting to cause light propagating in thewaveguide to emerge from one face of the waveguide, a lens element fordirecting light into an edge of the waveguide system, and in the focalplane of the lens element means for modulating the intensity of thelight as a function of lateral position, relative to the waveguide, thegrating being such as to eject light directed into the waveguide systemfrom the slab towards a viewer at an angle depending on the said lateralposition; in which the light emerging from the display slab is modulatedby an output panel to give a three-dimensional display.
 20. A flat-panelprojection display according to claim 19, in which the output panel is aliquid-crystal display panel mounted in front of the display slab, asseen by the viewer.
 21. A flat-panel projection display according toclaim 19, in which the output panel is a photosensitive reflector arraymodulated by a light source, and is mounted behind the display slab, asseen by the viewer.
 22. A flat-panel projection display according toclaim 21, in which the display slab contains a stack of partialreflectors reflecting the input light back towards the output panel andallowing to be reflected back again towards the viewer.
 23. A flat-panelprojection display comprising a waveguide system including a displayslab waveguide having a grating acting to cause light propagating in thewaveguide to emerge from one face of the waveguide, a lens element fordirecting light into an edge of the waveguide system, a magnifying slabwaveguide of matching width placed between the lens element and thedisplay slab waveguide so as to expand the light from the lens to thefull width of the display waveguide; a one-dimensional cylindricalmirror strip at the end of the magnifying waveguide, conserving out-ofplane angle but spreading light in the plane; and in the focal plane ofthe lens element means for modulating the intensity of the light as afunction of lateral position and of elevational direction of travel,relative to the waveguide, the light passing though the magnifying slabwaveguide in one direction for magnifying, being reflected by the mirrorstrip and passing back in the reverse direction for coupling into thedisplay waveguide; wherein the grating acts to eject light from themagnifying slab waveguide towards a viewer at an angle depending on thesaid lateral position and elevational direction.
 24. A flat-panelprojection display comprising a waveguide system including a displayslab waveguide having a grating acting to cause light propagating in thewaveguide to emerge from one face of the waveguide, a lens element fordirecting light into an edge of the waveguide system, and in the focalplane of the lens element means for modulating the intensity of thelight as a function of lateral position, relative to the waveguide, thegrating being such as to eject light directed into the waveguide systemfrom the slab towards a viewer at an angle depending on the said lateralposition; in which the light emerging from the display slab is modulatedby an output panel to give a three-dimensional display, the output panelbeing a liquid-crystal display panel mounted in front of the displayslab as seen by the viewer; and the means for modulating is a source ofcollimated light scanned through the range of output angles bytransverse movement or scanning of the light source in synchrony withthe modulating means.